Poly(3-hydroxyalkanoates) are naturally occurring thermoplastic polymers currently isolated from bacteria and other microorganisms. Unlike many thermoplastic polymers, poly(3-hydroxyalkanoates) and in particular poly(3-hydroxybutyrate) have been shown to be biodegradable and environmentally non-persistent. Poly(3-hydroxyalkanoates) have the additional feature of being both thermally and hydrolytically unstable and thus can degrade without microbial intervention. The high cost of processing and isolating poly(3-hydroxy-alkanoates) derived from natural sources however, has inhibited the wide spread use of these biodegradable polymers in commercial applications.
Poly(3-hydroxyalkanoates) can be prepared by the polymerization of beta-substituted-beta-propiolactones. A summary of this approach along with its strengths and weaknesses is disclosed in U.S. Pat. No. 5,281,691, the disclosure of which is incorporated herein by reference in its entirety. While potentially offering a low cost route to poly(3-hydroxyalkanoates), the preparation and polymerization of beta-substituted-beta-propiolactones has several problems which have hindered the commercialization of this process. While not well established, there is reason to be concerned over the toxicity of beta-substituted-beta-propiolactones. The possible hazards associated with beta-butyrolactone are disclosed in IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals in Man: Betabutyro-lactone, V11, 225 (1976). Significant expense could also be incurred in the manufacture of beta-substituted-beta-propiolactones. When producing isotactic polyhydroxybutyrate from beta-butyrolactone it is desirable to have a source high in (S)-beta-butyrolactone or (R)-beta-butyrolactone as discussed in U.S. Pat. No. 5,281,691 and Y. Hori et. al. in Macromolecules, V26, 5533 (1993). Significant expense is incurred in the manufacture of these optically pure or optically enriched monomers.
On the other hand, 3-hydroxyalkanoic acids and 3-hydroxyalkanoic esters are easily prepared from esters of acetoacetic acid and other sources and attempts have been made to use these as a raw material for the production of poly(3-hydroxyalkanoates) by condensation polymerization. However, as described by D. Seebach et. al., Angew. Chem. Int. Ed. Enql., V32, p. 487, (1993) and N. C. Billingham et. al., J. Organometal. Chem., V341, p 84, (1988) 3-hydroxybutyric acid and esters derived therefrom undergo elimination of the 3-hydroxy substituent under conditions of condensation polymerization. This undesirable elimination process thus seems to limit the molecular weight of condensation polymer to very low values.
In light of the above it would be very desirable to be able to synthetically produce poly(3-hydroxyalkanoates), particularly if it is by a low cost method. It would also be very desirable to be able to produce poly(3-hydroxyalkanoates) in a nontoxic process. It would further be very desirable to be able to produce crystalline poly(3-hydroxyalkanoates).